1. Technical Field
The invention relates to a planar light source. Particularly, the invention relates to a dual-side edge lighting type planar light source.
2. Related Art
In recent years, along with increase of light emitting efficiency and a service life of light emitting diodes (LEDs), and due to features and advantages of low power consumption, low pollution, high efficiency, high response speed, small size, light weight and capability of being configured on various surfaces, the LEDs are widely applied in various optical fields. Generally, the LEDs can be applied in illumination apparatus and flat panel displays (for example, liquid crystal displays (LCDs)).
Taking an application of the LEDs in the LCD as an example, the LEDs can be applied to a backlight module of the LCD for providing a uniform planar light source. A conventional LED backlight module includes a plurality of LEDs, a light guide plate and a reflector, etc., where the light guide plate may effectively convert the light emitted by the LED light source into a planar light source with a high luminance and good uniformity.
Presently, development of high luminance LEDs has become mature, when the high luminance LEDs are applied to the backlight module, the number of utilized LEDs can be effectively reduced. According to the existing technique, a design of disposing the LEDs at a corner of the light guide plate is provided, which may greatly decrease the number of the utilized LEDs to reduce the manufacturing cost.
Although the number of the high luminance LEDs used in the backlight module is relatively less, if an arrangement pitch between the adjacent LEDs is excessively large, hot spots may still appear at an edge of the light guide plate. In detail, in the known planar light source, the arrangement pitch of any two adjacent LEDs is P, and a shortest distance between a light emitting surface of each of the LEDs and an active area of the light guide plate is A. Generally, the manufacturer may optimize a ratio of A and P, i.e. an A/P ratio, according to a divergence angle of the LED. When the A/P ratio is lower, i.e. when the pitch P is excessively large/or the distance A is excessively small, a hot spot with lower luminance can appear at an edge of the active area (corresponding to a position between the LEDs), which influences uniformity of the planar light source. When the A/P ratio is higher, i.e. when the pitch P is excessively small/or the distance A is excessively large, the hot spot with lower luminance is not liable to appear at the edge of the active area.
Therefore, considering a demand on uniformity of the planar light source and reduction of manufacturing cost, how to eliminate the hot spots in the active area becomes an important issue in research and development of the planar light source.
China Patent No. 102506361A discloses an LED backlight module, in which a light source is disposed between a back plate and a light guide plate, where a light emitting surface of the light source faces to a light incident surface of the light guide plate, and is disposed in a certain angle between 0 degree and 90 degrees, and a groove is formed on the light guide plate at a position corresponding to the light emitting surface of the light source.
U.S. Pat. No. 8,267,563B2 discloses a light guide plate, in which a light incident surface includes a first light incident surface, a second light incident surface and a third light incident surface, where the first and the third light incident surfaces are symmetric to an intersection of a center line and a side of the light guide plate, and the light sources are also symmetric to the center line.
Japan Patent No. 4737409 discloses a light guide plate with point light sources disposed at a short side of the light guide plate, and two point light sources are disposed at a position that makes the emitted light diverging from a center line of a light emitting surface of the light guide plate.
China Taiwan Patent No. I314663 discloses a light guide plate, where a side of the light guide plate is designed in a groove shape concaved inward in collaboration with the number of LEDs to be used, and the light guide plate has light incident surfaces that are not in a same plane, and the LEDs are respectively disposed on the light incident surfaces in the grooves.
China Taiwan Patent No. M316410 discloses a light guide plate in which the light sources are disposed corresponding to a corner position of the light guide plate, and two concave surfaces concaved inwards into the light guide plate are formed at the corner position, where the concave surfaces are respectively a plane, and two light sources are disposed corresponding to each of the concave surfaces.